It may be force but there's no reason yet to think it is a fundamental force.
The question isn't can it be ruled out; the question is: is there sufficient cause to rule it in. i.e we don't invent new entities until the old ones are proven inadequate.
You don't need a fifth force for this. It's just gravity.
In General Relativity, the gravitational force depends not only upon energy density (in Newtonian mechanics, this is just the mass), but also upon pressure. With normal matter, which usually has negligible pressure compared to its mass energy, General Relativity describes gravity as we expect it to behave. If instead you have matter that has significant positive pressure, such as electromagnetic radiation (light), then it tends to gravitate and respond to gravity more strongly than we would naively expect (this is observed by the deflection of light waves by gravitational lensing being twice what we would expect from Newtonian gravity).
Well, with radiation, if you have a box full of radiation (this happens naturally if you just heat the box up), then that radiation will exert a pressure on the walls of the box equal to one third its energy density.
No. I'm saying that the radiation itself exerts a gravitational force. This is particularly important in the very early universe, when most of the energy density was in radiation. At that time, when our universe was dominated by radiation, the extra gravitational attraction due to the pressure of the radiation caused the expansion to slow even more rapidly than it did later, when the dominant energy was in matter.
Well, you can calculate it explicitly in General Relativity. Positive pressure = stronger attraction, expansion slows down faster. Negative pressure = weaker attraction, potentially even repulsion, causing the expansion to either slow down more slowly or speed up.
No, not at all. In General Relativity, gravity doesn't respond to mass. It responds to energy density, momentum density, pressure, and shear (twisting forces).
Well, some forms of matter are weird. The cosmological constant can be seen as a sort of matter with pressure that is equal to its energy density, only negative. A large number of quantum mechanical fields that are relatively easy to write down on paper (though we have no evidence for) also have similar negative pressure.
No, Chalnoth is then an accepted explanation for loud, vulgar eructations and noxious fumes but he is not an explanation for dark energy.
Well, there are many possible explanations. The simplest explanation that agrees with the current data is the cosmological constant, and as a result it is the leading explanation. But so far the evidence is far, far too sparse for anybody to say with any degree of confidence that the cosmological constant is the explanation.
In this analogy, consider the piston to be in a vacuum.
I agree it seems counter-intuitive. One would think that a negative pressure (i.e a tension pulling inwards) would lead to a contraction, not an expansion. All I can say is that when you analyze Einstein's field equations it leads to an expansion - i.e. your intuition is wrong, and we have to trust the mathematics. Perhaps someone else has a better explanation.
Well, most of the other explanations go under the name "quintessence", and are described by some sort of quantum scalar field with a particular potential. Wikipedia has a very rough description:
Basically, if you look at the derivation of Einstein's equations, there is no good reason to leave the constant term out. The reason why it has been left out for so long is because simple back-of-the-envelope calculations demonstrated that this term had to be incredibly small, around [itex]10^{-120}[/itex] in natural units, in order for our universe to exist at all. So people figured it must actually be set to zero by some symmetry or other. But nobody has ever found a symmetry that sets the cosmological constant to zero.
Yes, if there was enough mass relative to the current rate of expansion and no dark energy, or if the dark energy at some point in the future goes away and it turns out that the remaining mass is large enough compared to the rate of expansion, then it would recollapse. And in General Relativity, it would collapse to a singularity. Though to be fair, General Relativity cannot be trusted that far.
My impression is no, not if the universe didn't recollapse. My understanding of structure formation (admittedly basic understanding) is that in an expanding universe, matter that is within the Jeans' length collapses in on itself, while matter outside that length does not. So basically, structures will gobble up all the local matter they can, then stop growing. Dark energy turns off this growth sooner, but I think it stops in any expanding universe.
Think of an expanding universe like a fluid. If this fluid is made out of normal matter, then the gravity between the particles of the fluid wants to pull the whole thing together. So if it is expanding, gravity acts to slow the expansion by pulling everything closer to everything else. If this pull is strong enough, by the density being large compared to the expansion rate, then it will cause the universe to recollapse in on itself.